1. Field of the Invention
The present general inventive concept relates to a high power supply to control an abnormal load, more particularly, to a high power supply having an amplifying circuit formed in a single chip in an image forming apparatus, such as a laser printer or a laser multifunctional device, which detects an abnormal load at an output terminal using a load detecting sensor when the abnormal load is applied to the output terminal by a human resistance or a ground (GND) and interrupts an outputting operation of a high voltage by turning off the single chip amplifying circuit.
2. Description of the Related Art
Generally, a high power supply receives an input voltage of 5V or 24V from a switching mode power supply (SMPS) and a main board and generates a high voltage in response to a high/low signal of an engine controller or a pulse width modulation signal to form images during a transcription process. Also, the high power supply generates high voltage in a range of hundreds to thousands V using a transformer when receiving the 24V as the input voltage.
FIG. 1 is a circuit diagram illustrating a conventional high power supply,
Referring to FIG. 1, the conventional high power supply includes a controller 110, an input unit 120, an amplifier (OP-AMP) unit 130, a transformer 140, a rectifier 150, and an output unit 160.
The controller 110 supplies a PWM duty signal or an ON/OFF signal to the input unit 120 to output a high power signal. The input unit 120 converts the supplied power signal to a DC level signal using a low pass filter having a resistor R1 and a capacitor C1 by turning on a transistor TR1 upon receiving the ON signal and outputs the DC level signal with a reference signal to the amplifier unit 130.
The amplifier unit 130 includes an OP amp and resistors R2 and R3 and generates an input signal to the transformer 140 in order to control a level of an output voltage. The transformer 140 turns on a driving transistor (TR2) according to a signal from the input unit 120. The transformer 140 controls a base current of the driving transistor TR2 to control a time constant and an output level of an oscillatory circuit at an input side of the transformer and to generate an AC type voltage at an output side of the transformer.
The rectifier 150 includes diodes D1 and D2 and capacitors C2, C3, and C4, receives the AC type voltage from an output side of the transformer 140 and generates a DC type voltage, and the DC type voltage is outputted through the output unit 160. Here, an electric power is consumed by a load 170 connected to an output terminal of the output unit 150, and a portion of a current flowing between the rectifier 150 and the output unit 160 is fed back to the amplifier unit 130 through a resistor R4 and the feedback unit 180 before the current is inputted to the output unit 160.
As described above, the high voltage is used to form images by transferring a toner in an image forming apparatus having a high voltage power supply, and a roller of a developing unit is recognized as the load. Therefore, if the load, i.e., developing unit, is not included in the image forming apparatus, high voltage is not required to be generated. The generation of the high voltage may cause a serious safety problem in an abnormal situation such as when the developing unit is taken out from the image forming apparatus.
That is, when a user opens a cover of the image forming apparatus to take out the developing unit from the image forming apparatus, the user may touch a high voltage terminal. Accordingly, the input voltage must be interrupted when the cover is opened. If the user takes out the developing unit from the image forming apparatus when a cover opening switch is malfunctioned or in an abnormal situation, the user may receive electric shock by the exposed high voltage terminal.
According to a safety standard for the high power supply a human resistance is defined as 2 KΩ and is supplied at the output terminal when the high voltage is output, and a flowing current thereof is prevented to be exceeded 2 mA. However, if the output high voltage is shorted to the ground (GND) in an abnormal situation, it damages the electric circuit. Therefore, a safety device is required to prevent the high voltage when the output terminal is shorted to the ground or the human resistance.